feat: Add palette icon to Theme setting for better visibility

- Add Icons.palette leading icon to Theme ListTile in settings
- Language setting already had Icons.language icon
- Both appearance settings now have clear visual indicators
- Improves UX by making theme and language settings easier to identify

Author: aimuhire

mobile/lib/screens/settings_screen.dart

@@ -150,6 +150,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
                 child: Column(
                   children: [
                     ListTile(
+                      leading: const Icon(Icons.palette),
                       title: Text(AppLocalizations.of(context)!.theme),
                       subtitle: Text(_getThemeModeText(settings.themeMode)),
                       trailing: const Icon(Icons.arrow_forward_ios),